This invention relates to methods of making very thin, suspended layers of compound semiconductor materials, typically utilizing the GaAs/AlGaAs system. More particularly, this invention relates to methods of etching compound semiconductors to achieve thicknesses in the range of 10 microns or less for relatively large surface area layers. Still more particularly, this invention relates to methods of forming acoustic wave chemical microsensors and high frequency electronic filters made by these micromachining techniques.
GaAs and quartz have been used as the piezoelectric acoustic wave elements for chemical sensors for many years. If a coating that is selective for a chemical analyte of interest is placed on the surface of the piezoelectric element and the coating is then presented to a fluid mixture that may contain the analyte, the resonant frequency of the coated sensor will change as the analyte builds up on the element. One class of these sensors is of the type known as surface acoustic wave (SAW) sensors in which a relatively thick substrate layer of quartz or GaAs is utilized. In this class of sensors, the acoustic wavelength is small compared to the substrate thickness. The chemical sensitivity of the SAW device scales inversely with acoustic wavelength and is therefor greatest for the smallest possible wavelength. The SAW wavelength is determined by the width of and the spacing between the interdigitated electrodes used to drive the acoustic wave in the crystal. Therefor, the chemical sensitivity of the SAW device is limited by the resolution of the microlithography process that sets a lower limit on the acoustic wavelength. In commonly used configurations, the area occupied by this sensor scales as the square of the acoustic wavelength, decreasing as the wavelength decreases and the sensitivity increases. The acoustic frequency of this sensor is determined by the acoustic wavelength of the device and the acoustic velocity of the substrate material such that the frequency increases as the wavelength decreases and the sensitivity increases.
A second class of these sensors is of the type known as flexural plate wave (FPW) sensors. This class of sensors differs from SAW sensors in that the acoustic wavelength is comparable to or greater than the thickness of the substrate. For this class of sensors, the chemical sensitivity of the sensor increases as the thickness of the substrate decreases with constant acoustic wavelength. Therefor, the chemical sensitivity is limited by the ability to make thin substrates and is independent of the microlithography process used to form the interdigitated electrodes. The frequency of this device decreases for decreasing substrate thickness and increasing sensitivity. As with the SAW sensor, the area occupied by this sensor scales as the square of the wavelength.
A third class of these sensors is of the type known as thickness shear mode (TSM) sensors in which again a relatively thick substrate layer of quartz or GaAs is utilized in typical devices. In this class of sensors, the chemical sensitivity scales inversely with the thickness of the substrate, increasing for thinner substrates. As with the FPW sensor, the chemical sensitivity is limited by the ability to make thin substrates and is independent of the microlithography process used to form the electrodes. In commonly used configurations, the area occupied by this device scales inversely with the sensitivity, decreasing with increasing sensitivity and decreasing substrate thickness.
There exists a need in the art for a process to create thinner piezoelectric layers to increase chemical sensor sensitivity and to decrease area occupied by the sensor. In some applications, this need is coupled with an additional need to decrease the sensor frequency while in other applications this need is coupled with an additional need to increase the sensor frequency. Further, there exists a need in the art for a process to create acoustic wave chemical microsensors with increased performance in a manner that is compatible with the monolithic integration of microelectronic circuits that can control the sensors and extract data from them.
These same piezoelectric materials can also be used as signal processing and signal conditioning components in high frequency electronic circuit applications, particularly filters. The same structures used in microsensor devices, namely SAW, FPW, and TSM structures, provide the signal processing and conditioning function. In this case, the devices do not require the application of chemically selective layers. As with the sensors, the operating frequency for some of these devices will increase as the substrate thickness decreases and the area occupied by these devices will decrease as the substrate thickness decreases. As with the sensors, there exists a need in the art for a process to create thinner piezoelectric layers to increase the operating frequency and reduce the size of electronic filters. In addition, there exists a need for the monolithic integration of the improved filters with microelectronic circuits.